bpf: Fix a data-race around bpf_jit_limit.

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix a data-race around bpf_jit_limit.

While reading bpf_jit_limit, it can be changed concurrently via sysctl,

WRITE_ONCE() in __do_proc_doulongvec_minmax(). The size of bpf_jit_limit

is long, so we need to add a paired READ_ONCE() to avoid load-tearing.